In the background art, silicon single crystal has been used as a material for power semiconductor elements which control a high breakdown voltage and a large current. There are several types of power semiconductor elements. In the current circumstances, these types of power semiconductor elements are used properly in accordance with purposes. For example, a bipolar transistor or an IGBT (Insulated Gate Bipolar Transistor) can secure a large current density but cannot perform switching at high speed. The bipolar transistor has an application limit at several kHz while the IGBT has an application limit at about several tens of kHz.
On the other hand, a power MOSFET cannot secure a large current but can be used at high speedup to several MHz. However, due to a strong demand of the market for power devices that can secure a large current and high speed performance, efforts have been made to improve the IGBT and the power MOSFET so that development has been currently advanced up to the level substantially close to the material limit. In addition, material research has been made in view of a power semiconductor element. As a next-generation power semiconductor element, silicon carbide (hereinafter abbreviated to SiC) has gathered attention because it is an element excellent in low on-state voltage and high speed and high temperature characteristics (see NPL 1, identified further on).
SiC is a chemically very stable material. Due to the bandgap as wide as 3 eV, SiC can be used as a semiconductor extremely stably even at a high temperature. In addition, SiC is also larger in the breakdown strength by at least one digit than silicon. This also applies to gallium nitride (hereinafter abbreviated to GaN) which is another wide bandgap semiconductor material.
A Schottoky barrier diode having a rectification characteristic can be manufactured by depositing a metal on the surface of a wide bandgap semiconductor, similarly to silicon. From these reasons, it is possible to manufacture a Schottoky barrier diode with a high breakdown voltage and low on-resistance by using a wide bandgap semiconductor as a substrate material.
When a reverse voltage is applied to a diode having an ideal rectification characteristic, no current flows. When a forward voltage is applied to the diode, the diode has no resistance. However, when a reverse voltage is applied to a typically manufactured diode, a very small amount of current (leakage current) flows. When a forward voltage is applied to the diode, the diode has a little resistance (on-resistance). When a device having a Schottoky interface such as a Schottoky barrier diode has a high Schottoky barrier height, the device can restrain a leakage current to increase a breakdown voltage but on-resistance becomes large. On the contrary, when the Schottoky barrier height is low, the on-resistance becomes small but the leakage current becomes large.
As described above, there is a trade-off relation between the leakage current in the reverse electric characteristic and the on-resistance in the forward electric characteristic. From these reasons, a metal can be selected in accordance with a use purpose for manufacturing a Schottoky barrier diode. However, since the Schottoky barrier height of the manufactured Schottoky barrier diode is characterized by electron affinity of the semiconductor and the work function of the metal, an optimum Schottoky barrier diode for the use purpose cannot be always manufactured.
As described above, if the Schottoky barrier height is low, leakage increases even in a wide bandgap semiconductor Schottoky barrier diode. A diode using a junction barrier Schottoky structure (hereinafter abbreviated to JBS structure) is used as a method for solving this problem. In the JBS structure, a first conductive type semiconductor region under a Schottoky electrode is sandwiched between second conductive type semiconductor regions to deplete the first conductive type semiconductor in a Schottoky interface portion to thereby restrain a leakage current. In addition, as the thickness of a depletion layer (the width of a depletion layer extending from the Schottoky interface toward a semiconductor substrate) is larger, the leakage current is restrained more greatly.
As a general JBS structure, either a structure in which a first conductive type semiconductor forming a Schottoky interface and a second conductive type semiconductor are disposed alternately like stripes (see NPL 2, identified further on) or a structure in which a first conductive type semiconductor and a second conductive type semiconductor are disposed alternately and concentrically (see PTL 1, identified further on) has been used. However, since a depletion layer is thin in a Schottoky interface portion between a metal and the first conductive type semiconductor located away from the second conductive type semiconductor, an effect of reducing a leakage current is weakened. Moreover, when the distance between the second conductive type semiconductor regions which sandwich the first conductive type semiconductor is reduced for the purpose of widening the depletion layer in order to restrain the leakage current, on-resistance is increased.